Cationic surfactants

Cationic surfactants – characteristics and properties

Surfactants are amphiphilic compounds that contain both hydrophobic and hydrophilic groups, making them soluble in both organic solvents and water. Cationic surfactants are compounds that have a positively charged fragment in their structure.

The structure of cationic surfactants determines many of the functional properties that are desirable in industrial applications:

• When added to a system, they accumulate at the phase boundary and reduce surface tension, thus enabling rapid wetting of the surface.
• Interfacial action is also an important aspect of formulation stabilisation. Therefore, cationic surfactants act as emulsifiers in many mixtures.
• They have good antistatic properties. They are usually dispersed on the surface and their role is to increase surface conductivity, which helps to prevent the accumulation of surface charge.
• Cationic surfactants are used as corrosion inhibitors to protect metal materials from corrosion. The adsorption of the surfactant on metals and metal oxides creates a barrier that can inhibit corrosion.
• The dispersing action promotes the formation of fine droplets of one phase that disperse in the other phase. This reduces sedimentation and agglomeration, increasing the stability of the formulation.
• Some cationic surfactants are referred to as hydrotropes. This means that they increase the solubility of other components in the formulation.

Examples and classification of cationic surfactants

Key chemical groups of cationic surfactants include:

• Quaternary compounds: these are the most important and widely used group of cationic surfactants. They are formed by the reaction of tertiary amines with alkylating agents. They are characterised by good solubility and emulsifying properties, and are often used as wetting agents, emulsifiers and bactericides. Examples of cationic surfactants in the form of quaternary compounds include Chemquat 508/40,
• Fatty amine derivatives: these are obtained by the condensation of fatty acids (most commonly of animal origin) with amines. The PCC Group’s range includes, for example, ROKAmina K15K, a specialised surfactant from the group of alkoxylated fatty amines.

Criteria for selecting cationic surfactants

The hydrophilic-lipophilic balance (HLB) of a surfactant can be a useful indicator of the balance between hydrophilic and lipophilic groups, and thus indicates the properties of the surfactant. The HLB value is a key factor in the selection of formulations. It is generally accepted that, on a scale from zero to ten, a substance has a greater affinity for lipophilic molecules and therefore dissolves better in oils and hydrophobic compounds than in water. Above ten, however, the affinity for water and hydrophilic particles increases. Surfactants characterised by an HLB value >10 are therefore more hydrophilic, and their solubility increases in water whilst decreasing in oils.

In industry and practical applications, the ability of a given cationic surfactant to adsorb onto a surface is a key consideration. Quaternary ammonium salts perform particularly well in such applications. Possessing a permanent positive charge on the nitrogen atom, they are attracted to these surfaces by electrostatic forces. As a result, they can be used, for example, as anti-corrosion agents for steel, dispersants for inorganic pigments, fabric softeners, hair conditioners and anti-caking agents.

Cationic surfactants compared to other surface-active compounds

Although cationic surfactants are commonly used in detergents, they do not perform the same functions as their anionic, non-ionic or amphoteric counterparts. They are not usually chosen as primary cleaning agents, but are valuable additives to formulations. Compared to anionic and non-ionic compounds, cations are poor at removing dirt. Their primary function is to adhere to surfaces.

The cleaning ability of cationic surfactants is limited; however, their greatest advantage lies in their bactericidal properties, which clearly distinguish this group of surfactants. This characteristic makes them useful in the formulation of disinfectants, leaving a cationic disinfecting layer on the surface.

Furthermore, unlike amphoteric or non-ionic surfactants, which are compatible with most compounds, cationic surfactants cannot form an effective formulation with, for example, negatively charged molecules. The presence of strong positive and negative charges leads to a loss of solubility in the complexes and the formation of a precipitate.

Safety standards in surfactant technology

The placing on the market of chemical substances, such as cationic surfactants, and their use in industrial and laboratory processes requires strict compliance with applicable legal standards. The foundation of chemical safety is provided by the REACH and CLP regulations, which comprehensively regulate issues of registration, distribution and the correct labelling of products.

These regulations precisely define the scope of information required in the Safety Data Sheet, particularly regarding toxicity, biodegradation and exposure risks. Furthermore, the CLP Regulation mandates the use of standardised hazard pictograms.

In the cosmetics sector, cationic surfactants are subject to regulations concerning permissible concentrations and chemical purity, particularly in products that remain on the skin. Every raw material, prior to use in a formulation, should have a defined ‘safety profile’. Consumers are looking for products that meet ISO quality standards and – importantly in the context of cosmetic products – have not been tested on animals.

Equally stringent requirements apply to the use of cationic surfactants in the food industry. Although they are not used as direct food additives, they may still be components of packaging that comes into contact with food products. It is therefore essential to ensure their high quality and purity.

Application of cationic surfactants

Textiles

As a component of softening formulations, cationic surfactants are used in the textile industry to give finished products antistatic and softening properties. They are also used as one of the components dedicated to fibre dyeing. Cationic surfactants reduce the stiffness, roughness and coarseness of fabrics, making them soft and fluffy.

Cosmetics and detergents

Cationic surfactants are important conditioning ingredients in hair care cosmetics such as shampoos, regenerating conditioners and hairsprays. Their primary function is to improve combability, softness and reduce static electricity. In household chemicals, they are used in concentrates and fabric softeners, where they act as softeners. In addition, they are used in the antistatic treatment of industrial fibres.

Cationic surfactants are also an important ingredient in car care products. They are found in waxes and polishes, and are also an active component in rinses designed to speed up the drying of car bodies while preventing streaks and water spots.

Plastics

Cationic surfactants are used as additives in the production of plastics, primarily due to their antistatic properties. Their use in production is very important because the phenomenon of electrical charge accumulation limits the speed of the technological process, generates material losses, causes product contamination and accelerates its decomposition. Cationic surfactants are used, among other things, in the production of polyethylene or polypropylene films.

Paper industry

In paper production, cationic surfactants are included in softening formulations. During the production process, they give the finished products antistatic properties and have a softening effect.